The invention relates to an image sensor device for receiving a radiation image and converting it into an electrical signal. The device comprises at least one semiconductor body. A plurality of mutually separated, substantially parallel charge transfer channels are provided in the semiconductor body at a major surface thereof. An insulating layer is provided above the channels on the major surface. An electrode system is disposed on the insulating layer above the charge transfer channels for inducing charge transport. Windows are provided in the electrode system so that the radiation image can pass into the semiconductor body and can produce charge carriers therein. The windows are transparent to short wavelength light. The electrode system comprises a first group of electrodes which extend in a direction transverse to the charge transport channels.
Such a device is described in Dutch Patent Application No. 8000999 (corresponding to U.S. Pat. No. 4,463,367). In the radiation-sensitive part of this device, a pattern of charge packets is produced which corresponds to the radiation or exposure pattern. After the end of the integration period, the charge packets can be transferred to a storage register (frame/field transfer). The charge stored in the storage register is then shifted line by line into a shift register, from which it can be read for further processing.
By means of the electrode system, depletion regions can be induced in the underlying semiconductor body. In or near these depletion regions, charge carries can be produced by absorption of radiation. These charge carriers can then be stored in the depletion regions below the electrode system in the form of the aforementioned charge packets.
Dutch Patent Application 8000999 shows an image sensor device in which the windows are bounded transversely to the direction of charge transport by the first group of electrodes. The windows are bounded in the direction of transport by parts of an electrode which mainly extend above the transport channels. In this manner, the image sensor device can shift the charge packets produced in the radiation-sensitive part into the storage register by means of a three phase clock system. In order to obtain a shorter response time and a high speed of transport, the electrode elements parallel to the direction of transport may be interconnected, if desired, by transverse connections. Such an electrode structure provides an image sensor device with a very high sensitivity, especially to blue light.
For further processing, the stored charge which corresponds to a sensed image is generally converted into a signal for a television receiver. To do this, the charge packets in the even numbered lines are read out to form a first field, and the charge packets in the odd numbered lines are read out to form a second field. It is therefore desirable for this so-called interlacing that within one frame period (1/30-1/25 second) charge packets are transferred twice from the radiation-sensitive part to the storage register. The charge storage takes place alternately in different parts of the radiation-sensitive part. For this purpose, the charge packets are alternately collected in the image sensor device within one image period at different areas, i.e. alternately below the two electrodes of the first group and below the electrode parts limiting the windows in the direction of transport.
However, problems arise with this method of interlacing. One problem is that, in order to obtain a high radiation sensitivity, the parts located between the windows of the electrodes parallel to the direction of transport have lengths exceeding their widths. The widths of these electrodes will generally be the minimum track width of the conductor tracks used due to minimization of the surface area to be utilized. In order to obtain a high radiation sensitivity, the lengths of these parts is chosen a few times larger (in a typical embodiment, for example 14 .mu.m with a minimum track width of 3.6 .mu.m). When charge is collected under these electrodes, the associated depletion regions therefore have surface areas of approximately 50 .mu.m.sup.2. At the same time, the integration regions are accurately defined due to the fact that the relative distances between the electrodes is not excessively large (approximately 5 .mu.m) so that electrons produced between the transfer channels are distributed over these adjacent transport channels.
In the device described in Dutch Patent Application No. 8000999, the channel stopper regions extend under the electrodes of the first group and directly limit the transfer channel. During a second integration period, charge is collected within the same image period under the electrodes which are located at right angles to the charge transport channel and overlap each other in part. With the same (minimum) track width, in this case the depletion region would occupy at most a surface area of approximately 12 .mu.m.sup.2. Moreover, the depletion region thus produced adjoins the surrounding radiation-transparent windows only at the corners. This means that the charge carriers, especially when they are produced at the center of such a window, must travel across an additionally long path to the depletion region as compared with the situation during the aforementioned first integration period. This effect is increased further by the fact that, in the direction of transport the windows are longer than in the transverse direction.
In order to avoid this effect, the signals produced could be amplified differently, but this requires the use of additional control electronics.
The enlargement of the depletion region under the first electrodes by choosing narrower channel stopper regions under these electrodes, is also only partly effective. In fact, it has been found that especially due to the large distance from the electrode parallel to the direction of transport, the effective sensitivity of adjacent integration regions can be considerably affected by process variations and deviations in the geometry of the electrodes. This is because such variations cause the barriers under these electrodes to be poorly defined. This could possibly be improved slightly by electronically averaging the two image signals in signal processing.